ACOUSTICS Designer’s NOTEBOOK Fig. 1 Sound transmission class as a function of wall weight.

ACOUSTICS 65

General 60 The basic purpose of architectural acoustics is to provide a satisfactory Flat or Ribbed Panels environment in which the desired sounds are clearly heard by the intended 55 listeners and the unwanted sounds (noise) are isolated or absorbed. The sound-reduction needs of a building are determined based on location, environmental ambiance, and the degree of sound reduction necessary for 50 occupants to function effectively. STC = 0.1304 W + 43.48 Under most conditions, the architect can design the building to satisfy Statistical Tolerance ± 2.5 STC 45 the acoustical needs of the tenant. Good acoustical design uses reflective Class (STC) Transmission Sound and absorptive surfaces, sound barriers, and vibration isolators. Some surfaces must reflect sound so that the loudness will be adequate in all 40 areas where listeners are located. Other surfaces must absorb sound 40 50 60 70 80 90 100 to avoid echoes, sound distortion, and long reverberation times. Sound is Weight Per Unit Area - (W) - psf isolated from rooms where it is not wanted by selected wall and floor/ceiling constructions. Vibrations generated by mechanical equipment are isolated from the structural frame of the building by means of mechanical isolators or compressible materials. Fig. 2 Most acoustical situations can be described in terms of: (1) sound Acoustical test data of solid flat concrete panels – source, strength, and path; (2) sound transmission path; and (3) sound normalweight concrete. receiver. Sound Transmission Loss Sound Levels The problems of sound insulation are usually considerably more 70 complicated than those of sound absorption. Sound insulation involves greater reductions in sound level than can be achieved by absorption. These 8 in. Flat Panel, STC-58 large reductions can only be achieved by continuous, impervious barriers. If 60 the problem also involves structure-borne sound, it may be necessary to 6 in. Flat Panel, STC-55 introduce resilient layers or discontinuities into the barrier. Sound absorbing materials and sound insulating materials are used for two different purposes. There is not much sound absorption from an 8 in. 50 (200 mm) concrete wall; similarly, low sound transmission is not available from a porous, lightweight material that may be applied to room surfaces for sound absorption. It is important to recognize that the basic mechanisms 4 in. Flat Panel, STC-49

Sound Transmission Loss dB Transmission Sound 40 of sound absorption and sound insulation are quite different.

Sound Transmission Loss 30 125 315 100 160 250

Sound transmission loss measurements are made at 16 frequencies at 200 400 500 630 800 1250 3150 1000 1600 2500 2000 4000 one-third octave intervals covering the range from 125 to 4000 Hz. The 5000 testing procedure is described in ASTM E 90, Laboratory Measurement of Frequency, HZ Airborne Sound Transmission Loss of Building Partitions. Measurements can also be made in buildings by following ASTM E 336, Measurement of Airborne Sound Insulation in Buildings. To simplify specification of desired Precast concrete walls usually do not need additional treatments in order to performance characteristics the single number Sound Transmission Class provide adequate sound insulation. If desired, greater sound insulation can (STC) (see ASTM E 413) was developed. It was originally designed to assess be obtained by using a resiliently attached layer(s) of gypsum board or other sound (human speech) privacy for interior walls, but its use has expanded to building material. The increased transmission loss occurs because the energy cover virtually all types of partitions and partition elements. flow path is increased to include a dissipative air column and additional mass. Airborne sound reaching a wall, floor, or ceiling produces vibrations in the The acoustical test results of airborne sound transmission loss of 4, 6, wall that are radiated with reduced intensity on the other side. Airborne sound and 8 in. (100, 150, and 200 mm) solid flat panels are shown in Fig. 2. Table 1 transmission loss in wall assemblies is a function of their weight, stiffness, presents the ratings for various precast concrete assemblies. The effects of and vibration damping characteristics. various assembly treatments on sound transmission can also be predicted Weight is concrete’s greatest asset when it is used as a sound insulator. from results of previous tests shown in Table 2. The improvements are additive, For sections of similar design, but different weights, the STC increases but in some cases the total effect may be slightly less than the sum. approximately 6 units for each doubling of weight (Fig. 1). This figure describes The mass of the precast/prestressed concrete loadbearing sandwich wall sound transmission class as a function of weight based on experimental data. panels prevented outside noises from entering the building in Fig. 3. The design 40 Table 1–Airborne Sound Transmission Class Ratings from Tests of Precast Concrete Assemblies.

Assembly STC1 No. Description (OITC) 1 4 in. flat panel, 54 psf 49 (43) 2 5 in. flat panel, 60 psf 522 Acceptable Noise Criteria As a rule, a certain amount of continuous sound can be tolerated before 3 6 in. flat panel, 75 psf 55 (46) it becomes noise. An “acceptable” level neither disturbs room occupants nor Assembly 2 with “Z” furring interferes with the communication of wanted sound. 1 4 channels, 1 in. insulation and /2 in. 62 gypsum board, 75.5 psf The most generally accepted noise criteria (NC) used today are expressed as

1 the Noise Criteria or the Room Criteria (RC) curves (Fig. 4, Table 3 and Fig. 5). Assembly 2 with wood furring, 1 /2 1 5 in. insulation and /2 in. gypsum 63 The figures in Table 4 represent general acoustical goals. They can also be board, 73 psf compared with anticipated noise levels in specific rooms to assist in evaluating

1 noise-reduction problems. Assembly 2 with /2 in. space, 5 1 2 6 1 /8 in. metal stud row, 1 /2 in. 63 The main criticism of NC curves is that they are too permissive when the 1 insulation and /2 in. gypsum board control of low or high frequency noise is of concern. For this reason, room 7 8 in. flat panel, 95 psf 58 (50) criteria (RC) curves were developed (Fig. 5). RC curves are the result of extensive studies based on the human response to both sound-pressure level and 8 10 in. flat panel, 120 psf 592 frequency and take into account the requirements for speech intelligibility.

1 The STC of sandwich panels is about the same as the STC of the thickness of A low background level obviously is necessary where listening and speech the two concrete wythes (ignoring the insulation thickness). intelligibility is important. Conversely, higher ambient levels can persist in 2 Estimated values. large business offices or factories where speech communication is limited to short distances. Often, the minimum target levels are just as important as the maximum permissible levels listed in Table 4. In an office or residence, it is Table 2–Typical Improvements for Wall Treatments Used with desirable to have a certain ambient sound level to assure adequate acoustical Precast Concrete Elements. privacy between spaces and minimize the transmission loss requirements of unwanted sound (noise). Increased These undesirable sounds may be from exterior sources such as automobiles Airborne and aircraft, or they may be generated as speech in an adjacent classroom or Treatment STC music in an adjacent apartment. They may also be direct impact-induced sound 3 1 Wall furring, /4 in. insulation and /2 in. gypsum such as footfalls on the floor above, rain on a lightweight roof construction, or 3 board attached to concrete wall vibrating mechanical equipment. Thus, the designer must always be ready to

1 accept the task of analyzing the many potential sources of intruding sound as Separate metal stud system, 1 /2 in. insulation in 1 related to their frequency characteristics and the rates at which they occur. stud cavity and /2 in. gypsum board attached to 5 to 10 concrete wall The level of toleration that is to be expected by those who will occupy the space must also be established. Figures 6 and 7 are the spectral characteristics of Plaster direct to concrete 0 common noise sources. With these criteria, the problem of sound isolation now must be solved, of this auditorium required selected areas of high resolution and reflectivity, namely the reduction process between the high, unwanted noise source and the which was achieved by using the 8 in.-thick (200 mm) curved interior wall panels desired ambient level. Once the objectives are established, the designer then to distribute sound throughout the hall in a geometrically controlled fashion. should refer to available data (for example in Fig. 1 or Table 1) and select the They also serve as structural members. Some 200 curved, sandblasted panels, system that best meets these requirements. In this respect, precast concrete employing eight different radii, were created to meet all of the acoustical systems have superior properties and can, with minimal effort, comply with requirements. They were given a staining sealer for aesthetic effects. these criteria. When the insulation value has not been specified, selection of the necessary barrier can be determined analytically by (a) identifying exterior and /or interior noise sources, and (b) by establishing acceptable interior noise Absorption of Sound criteria. A sound wave always loses part of its energy as it is reflected by a surface. Example: Sound Insulation Criteria This loss of energy is called sound absorption. It appears as a decrease in Assume a precast concrete office building is to be erected adjacent to a sound pressure of the reflected wave. The sound absorption coefficient is the major highway. Private and semiprivate offices will run along the perimeter of the fraction of energy incident but not reflected per unit of surface area. Sound structure. The first step is to determine the degree of insulation required of the absorption can be specified at individual frequencies or as an average of exterior wall system (see Sound Pressure Level 1, page 44). The NC data is used absorption coefficients (NRC). A dense, non-porous concrete surface typically because it is more familar to and preferred by designers. absorbs 1 to 2% of incident sound and has an NRC of 0.015. In cases where additional sound absorption is desired, a coating of acoustical material can The 500 Hz requirement, 38 dB, can be used as the first approximation of be spray-applied, acoustical tile can be applied with adhesive, or an acoustical the wall STC category. However, if windows are planned for the wall, a system of ceiling can be suspended. Most of the spray-applied fire-retardant materials about 50–55 STC should be selected (see following composite wall discussion). used to increase the fire resistance of precast concrete and other floor-ceiling Individual transmission loss performance values of this system are then systems can also be used to absorb sound. The NRC of the sprayed fiber types compared to the calculated need (see Sound Pressure Level 2). range from 0.25 to 0.75. Most cementitious types have an NRC from 0.25 to The selected wall should meet or exceed the insulation needs at all 0.50. frequencies. However, to achieve the most efficient design conditions, certain 41 (b)

Precast concrete controls the acoustics.

Table 3–Data for noise criteria curves.

Noise Criteria Octave Band Center Frequency, Hz Curves 63 125 250 500 1000 2000 4000 8000 NC-151 47 36 29 22 17 14 12 11 NC-201 51 40 33 26 22 19 17 16 (a) NC-251 54 44 37 31 27 24 22 21 NC-30 57 48 41 35 31 29 28 27 Fig. 3 NC-35 60 52 45 40 36 34 33 32 The Juanita K. Hammons Hall for the Performing Arts, Springfield, Missouri; NC-40 64 56 50 45 41 39 38 37 Architect: Pellham-Phillips-Hagerman NC-45 67 60 54 49 46 44 43 42 and Butler, Rosenbury & Partners (joint venture); NC-50 71 64 58 54 51 49 48 47 Photos: Pellham-Phillips-Hagerman. NC-55 74 67 62 58 56 54 53 52 NC-60 77 71 67 63 61 59 58 57 NC-65 80 75 71 68 66 64 63 62

1 The applications requiring background levels less than NC-25 are special pur- Fig. 4 Noise criteria (NC) curves. pose spaces in which an acoustical consultant should set the criteria. 90

limited deficiencies can be tolerated. Experience has shown that the maximum 80 deficiencies are 3 dB at two frequencies or 5 dB on one frequency point.

70 Composite Wall Considerations An acoustically composite wall is made up of elements of varying acoustical NC-65 60 properties. Windows and doors are often the weak link in an otherwise NC-60 effective sound barrier. Minimal effects on sound transmission loss will be NC-55 achieved in most cases by proper selection of glass (Table 5). The control of 50 sound transmission through windows requires large cavities between layers NC-50 (multiple glazing), heavy layers (thicker glass), laminated glass, and reduction NC-45 of the structural connection between layers (separate frames and sashes 40 NC-40 for inner and outer layers). Also, mounting of glass lites with soft neoprene NC-35 edge gaskets may not be as effective at reducing sound transmission as 30 systems that use wet seals (gunable sealants). The combination of wet seals NC-30 with butyl tape or open cell foam dramatically reduces the potential for air Approximate NC-25 infiltration, and therefore, flanking sound transmission. They certainly have to 20 threshold of NC-20 be as airtight as possible; usually fixed windows provide much better sound hearing for continuous NC-15 transmission control than operable windows. noise Octave Band Sound Pressure Level, dB re 20 micropascals dB re Level, Pressure Sound Band Octave 10 Sound pressure impinging on the window framing will cause it to vibrate, 16 125 250 500 1000 2000 4000 8000 transmitting sound to the building interior. Consequently, the window-glass Octave Band Center Frequencies, Hz performance cannot solely be relied on to reduce sound transmission to the building interior. The sound transmission of the window framing will result in such as mechanical system or transportation noise. The OITC (Outdoor- higher levels of sound transmission through the glass and wall. Also, window- Indoor Transmission Class) rating system based on ASTM E 1332 is relatively framing systems that allow greater amounts of air infiltration also allow new, and it was designed to assess a building façade element, such as a greater sound transmission. window, when exposed to a standard spectrum of low frequency air and truck STC is not necessarily the best performance specification for windows transportation noise ranging from 80 to 4000 Hz (see ASTM Guide E 966). as it is often a poor predictor of sound insulation for low frequency sources, Therefore, it is a better measure of a window system’s performance than 42 Table 4–Recommended Category Classification and Suggested Noise Criteria Range for Steady Background Fig. 5 Room criteria (RC) curves. Noise as Heard in Various Indoor Functional Activity Areas.1 90 NC or RC A Type of Space Curve 80 1. Private residence 25 to 30 B 70 2. Apartments 30 to 35

3. Hotels/motels 60 a. Individual rooms or suites 30 to 35 b. Meeting/banquet rooms 30 to 35 50 RC c. Halls, corridors, lobbies 35 to 40 50 40 C d. Services/support areas 40 to 45 45 40 4. Offices 30 35 a. Executive 25 to 30 20 30 b. Conference rooms 25 to 30 25

c. Private 30 to 35 20 micropascals dB re Level, Pressure Sound Band Octave 10 16 31.5 63 125 250 500 1000 2000 4000 d. Open-Plan areas 35 to 40 Octave Band Center Frequencies, Hz e. Computer/business machine areas 40 to 45 Region A: High probability that noise-induced vibration levels in lightweight wall/ceiling constructions will be clearly f. Public circulation 40 to 45 perceptible; anticipate audible rattles in low mass fixtures, doors, windows, etc. 5. Hospitals and clinics Region B: Noise-induced vibration levels in lightweight wall/ a. Private rooms 25 to 30 ceiling constructions may be moderately perceptible; slight possibility of rattles in low mass fixtures, b. Wards 30 to 35 doors, windows, etc. Region C: Below threshold of hearing for continuous noise. c. Operating rooms 25 to 30 d. Laboratories 30 to 35 e. Corridors 30 to 35 The sound-transmission loss through a door depends on the material and construction of the door and the effectiveness of the seal between the door f. Public areas 35 to 40 and its frame. There is a mass law dependence of STC on weight (psf) for both 6. Churches 25 to 302 wood and steel doors. The approximate relationships are: 7. Schools For steel doors: STC = 15 + 27 log W a. Lecture and classrooms 25 to 30 For wood doors: STC = 12 + 32 log W b. Open-Plan classrooms 30 to 352 where W = weight of the door, psf. These relationships are purely empirical and a large deviation can be 8. Libraries 30 to 35 expected for any given door. ASTM E 1408 can be used to determine the 9. Concert halls2 acoustical performance of doors. 10. Legitimate theaters2 For best results, the distances between adjacent door and/or window openings should be maximized, staggered when possible, and held to a 2 11. Recording studios minimum area. Minimizing openings allows the wall to retain the acoustical 12. Movie theaters 30 to 35 properties of the precast concrete. The design characteristics of the door or window systems must be analyzed prior to specification. Such qualities as

1 Design goals can be increased by 5dB when dictated by budget constraints or frame design, door construction, and glazing thickness are vital performance when noise intrusion from other sources represents a limiting condition. criteria. Installation procedures must be exact and care should be given to the 2 An acoustical expert should be consulted for guidance on these critical spaces. framing of each opening. Gaskets, weatherstripping, and raised thresholds serve as both thermal and acoustical seals and are recommended. STC, especially when traffic noise is the principal concern. The numeric value Figure 8 can be used to calculate the effective acoustic isolation of a wall representation of OITC tends to be lower than the STC rating. system that contains a composite of elements, each with known individual There are many options available for acoustical glazing, so it is important transmission loss data (TL). (For purposes of approximation, STC values can to make the right choice—especially if the building is exposed to significant be used in place of TL values.) exterior noise and the interior spaces are noise sensitive. The use of double- pane insulating glass is not adequate for many projects. Even single- or Example: Composite Wall Insulation Criteria double-laminated insulating glass may not be adequate, especially at To complete the office building wall acoustical design from page 41 assume low outside temperatures, where regular PVB-laminated glass will yield a the following: performance similar to that of non-laminated glass. 1. The glazing area represents 10% of the exterior wall area. 43 Fig. 6 Sound pressure levels — exterior noise sources. Fig. 7 Sound pressure levels — interior noise sources.

120 Jet Aircraft Takeoff 120 500 ft Riveting 100 100 Bus Stereo Phnograph, Propeller Aircraft Teenager Lever Typical Office 80 Takeoff 500 ft Heavy Truck – 20 ft 80

60 60 Business Machine Automobiles – 20 ft Tabulating Room

40 Bed or Dining Room

Sound Pressure Level, dB Level, Pressure Sound 40 Sound Pressure Level, dB Level, Pressure Sound Kitchen

20 20

0 0 63 125 250 500 1000 2000 4000 8000 63 125 250 500 1000 2000 4000 8000 Frequency, Hz Frequency, Hz

Sound Pressure Level 1. 2. The windows will be double glazed with a 40 STC acoustical Sound Pressure Level – (dB) insulation rating. Frequency (Hz) 63 125 250 500 1000 2000 4000 8000 The problem now becomes the test of determining the combined effect of the concrete-glass combination and a re-determination of Bus traffic source 80 83 85 78 74 68 62 58 criteria compliance (see Sound Pressure Level 3). noise (Fig. 6) The maximum deficiency is 3 dB and occurs at only one frequency Private office noise point. The 6 in. (150 mm) precast concrete wall with double-glazed criteria – NC 35 60 52 45 40 36 34 33 32 windows will provide the required acoustical insulation. (Fig. 4) Floor-ceiling assembly acoustical insulation requirements are Required insulation 20 31 40 38 38 34 29 26 determined in the same manner as walls by using Fig. 2 and 8. Leaks and Flanking Sound Pressure Level – (dB) Performance of a building section with an otherwise adequate STC can be Frequency (Hz) 125 250 500 1000 2000 4000 seriously reduced by a relatively small hole (or any other path) that allows sound to bypass the acoustical barrier. All noise that reaches a space Required insulation 31 40 38 38 34 29 by paths other than through the primary barrier is called flanking noise. 6 in. precast Common flanking paths are openings around doors or windows, electrical concrete solid 38 43 52 59 67 72 outlets, telephone and television connections, and pipe and duct penetrations. concrete wall (Fig. 2) Suspended ceilings in rooms where walls do not extend from the ceiling to the Deficiencies ------roof or floor above also allow sound to travel to adjacent rooms by flanking. Anticipation and prevention of leaks begins at the design stage. Flanking Sound Pressure Level 2. paths (gaps) at the perimeters of interior precast concrete walls and floors Sound Pressure Level 3. are generally sealed during construction with grout or drypack. All openings Sound Pressure Level – (dB) around penetrations through walls or floors should be as small as possible and Frequency (Hz) 125 250 500 1000 2000 4000 must be sealed airtight. The higher the required STC of the barrier, the greater the importance of sealing all openings. 6 in. precast solid concrete wall (Fig. 2) 38 43 52 59 67 72 Perimeter leakage commonly occurs at the intersection between an exterior cladding panel and a floor slab. It is of vital importance to seal this gap to Double-glazed windows retain the acoustical integrity of the system and provide the required fire (Table 5) 17 33 40 41 40 54 stop between floors. One way to seal the gap is to place a 4 pcf (64 kg/m3) Correction (Fig. 8) 10 3 4 9 16 9 density mineral wool blanket between the floor slab and the exterior wall. Combined transmission loss 28 40 48 50 51 63 Figure 9 demonstrates the acoustical isolation effects of this treatment. An enhancement to Fig. 9 would be to recess the insulation below the floor plane Insulation requirements 31 40 38 38 34 29 and fill the recess with smoke stop elastomeric sealant. Thereby improving not Deficiencies - 3 — — — — — only the sound but the smoke resistance of the assembly. Flanking paths can be minimized by: Note: 1 in. = 25.4 mm 1. Interrupting the continuous flow of energy with dissimilar materials, that 44 Table 5–Acoustical Properties of Glass. Sound Transmission Class (STC) Type and Overall Thickness, Construction Space, in. Inside Lite, in. in. Outside Lite, in. STC OITC

5 1 3 1 /8 Insulated Glass /8 /8 /8 31 26

1 1 /4 Plate or Float — — /4 31 29

1 1 /2 Plate or Float — — /2 36 32

1 1 1 1 Insulated glass /4 /2 Air space /4 35 28-30

1 1 1 /4 Laminated /8 0.030 Vinyl /8 35 —

1 1 9 3 1 /2 Insulated glass /4 /16 Air space /16 37 28-30

3 3 /4 Plate or Float — — /4 36 —

1 1 1 1 Insulated glass /4 Laminated /2 Air space /4 39 31 1 Plate or Float — — 1 37 —

3 1 1 2 /4 Insulated glass /4 2 Air space /2 39 —

1 1 1 1 1 Laminated Insulated glass /4 /2 Air space /8 plus /8 41 32 Transmission loss (dB) Frequency (Hz) 125 160 200 250 315 400 500 630 800 1000 1250 1600 2000 2500 3150 4000

1 /4 in. plate glass – 31 STC; 29 OITC 25 25 24 28 26 29 31 33 34 34 35 34 30 27 32 37

1 1 in. insulating glass with /2 in. air space – 35 STC; 28 OITC 24 29 22 22 25 30 33 35 38 40 42 42 37 37 43 46

1 1 in. insulating glass laminated with /2 in. air space – 39 STC; 31 OITC 17 28 29 33 34 38 40 40 41 41 41 41 40 43 49 54

Fig. 8 Chart for calculating the effective transmission Fig. 9 Effect of safing insulation seals. loss of a composite barrier. 6 in. Concrete Floor 55 STC Sealant Inorganic Mineral 1 Wool Insulation

2 Percent of total area of ≥ 1/8 in. Steel Bent Plate wall occupied by door, Gap Exterior Wall 100 window or opening 5 50

20 10 Combined 10 Trasmission Loss 5 15 2 No closure 14 STC 20 1 0.5 With steel bent plate closure 28 STC 0.2 30 0.1 With 4 in. thick safing insulation 30 STC TL (Wall) – TL (Door, Window or Opening) – TL (Door, TL (Wall) 40 steel bent plate added 42 STC 50 60 With 6 in. thick safing insulation 38 STC 1 2 3 4 5 6 7 8 9 10 15 20 30 40 50 60 steel bent plate added 45 STC Decibels to be Subtracted from TL of Wall for Effective TL of Composite Barrier propensity to transmit flanking sound. In other words, the probability of existing flanking paths in a concrete structure is much less than in a is, expansion or control joints or air gaps. structure with steel or wood framing. 2. Increasing the resistance to energy flow with floating floor systems, full If the acoustical design is balanced, the maximum amount of acoustic height and/or double partitions, and suspended ceilings. energy reaching a space via flanking should not equal the energy transmitted 3. Using primary barriers, which are less subject to the creation of flanking through the primary barriers. In exterior walls, the proper application of paths. Although not easily quantified, an inverse relationship exists sealant and backup materials in the joints between units will not allow sound between the performance of an element as a primary barrier and its to flank the wall. 45